Telomere molecular biology is far more complex than originally thought; understanding it is aided by study of evolutionary variants, and Drosophila telomeres are remarkable variants: Drosophila lack telomerase. Instead, Drosophila telomeres are long tandem arrays of two non-LTR retrotransposons, HeT-A and TART, not the arrays of simple repeats generated by telomerase in almost all other organisms. HeT-A and TART are the first transposable elements found with a bona fide role in cell structure; thus, they offer opportunity for better understanding of both telomeres and non-LTR retrotransposons, a major class of retroelements. Having discovered that D.virilis has transposon telomeres, one of our major goals is to use the approximately 60 MY divergence between this species and D. melanogaster to find conserved features in transposon structure and interactions within their host cells by sequence analysis and by cross-species cell biological interaction assays. Another major goal derives from our D. melanogaster studies that provided the first information on the intracellular Iocalizations of Gag proteins from non-LTR retrotransposons and have identified proteins that may be involved in the localization of HeT-A and TARTGags, both at the telomere and in transit. We will use molecular studies to determine the involvement of these proteins with Gag and/or telomere DNA. We wilt also use molecular techniques to study proteins whose telomeric association was detected by genetic analysis. Thus, we now propose (1) to compare the sequences and cell biology of telomere transposons from distantly related species to deepen insight into the evolution of transposon telomeres, (2) to understand the significance of the developmentally regulated transcription of telomere transposons by using RNAi to specifically deplete transcripts and thereby determine which cell types show phenotypic effects and then to study those effects, (3) to analyze the cell biology of retrotransposon targeting to telomeres, using genetic and biochemical techniques to determine the molecular interactions of proteins involved in determining the path of the retrotransposon from the cytoplasm to its target at the end of the chromosome, and (4) to compare the chromatin formed by telomeric transposon arrays with that found in other telomeres.